1. Field of the Invention
The present invention relates generally to an injection molding die for injection-molding base boards usable for an information recording medium such as a photoelectromagnetic disc or the like. More particularly, the present invention relates to an improvement of an injection molding die of the foregoing type wherein the injection molding die not only improves the accuracy of a double refraction index by increasing a filled density of each molded product, but also improves an accuracy of a formation of a center hole through each molded product.
2. Description of the Related Art
In practice, there ate two problems which are solved by the present invention relating to an injection molding die for injection-molding base boards usable for an information recording medium such as a photoelectromagnetic disc or the like.
One problem is concerned with further improvement of an accuracy of the dimensions of each molded product, as well as an accuracy of a center hole formed by a die component through each molded product, compared with a conventional injection molding die of the foregoing type.
The other problem is concerned with further improvement of an accuracy of a double refraction index of a base board as well as stamper pattern transference to each molded product.
To facilitate understanding of the present invention, typical conventional injection molding dies of the foregoing type will briefly be described below with reference to FIG. 4 to FIG. 7.
FIG. 4 is a fragmentary sectional view of a conventional injection molding die for injection-molding base boards as disclosed by Japanese Patent Application Laid-Open Publication No. 60-23970 (corresponding to U.S. Pat. No. 4,372,741), particularly illustrating a problem which arises when a center hole is formed through each base board in the injection molding die.
Specifically, FIG. 4 illustrates the operative state of the left-hand side L of the injection molding die wherein a molten resin 1 fills a cylindrical die cavity defined between a die half 2 on a stationary side, and a die half 3 on a movable side. The resin 1 is injected through an injection hole 5 formed through a nozzle bush 4 extending along the center axis of the injection molding die.
In addition, FIG. 4 illustrates the operative state of the right-hand side R of the injection molding die wherein a sleeve valve 6 is brought in contact with the lowermost end of nozzle bush 4 at the substantially same time the die cavity is fully filled with the molten resin 1, so that further feeding of the molten resin is instantaneously interrupted, and moreover, a center hole is formed through the fully filled layer of molten resin. In this case, an extra quantity of molten resin discharged from the die cavity by the upward displacement of sleeve valve 6 is permitted to freely flow only in the space formed by the downward displacement of a core pin 7, but is not permitted to flow outside of the outer peripheral surface of the sleeve valve 6. Thus, there arises a problem that the center hole exhibits a coarse appearance similar to that when formed by employing a punching process. Usually, an annularly extending convex plane is formed around the center hole as represented by reference character a.
Next, FIG. 5 is a fragmentary sectional view of another conventional injection molding die for injection-molding base boards as disclosed in U.S. Pat. No. 3,989,436, particularly illustrating a problem which arises when a center hole is formed through each base board. In the drawing, reference numeral 8 designates a die half on the stationary side of the injection molding die. Reference numeral 9 designates a die half on the movable side, and reference numeral 10 designates a hole cut punch. Reference numeral 11 designates a nozzle bush, and reference numeral 12 designates a molded product of base board.
Specifically, FIG. 5 illustrates the operative state of the left-hand side L of the injection molding die before a center hole is formed through a molded product of base board by actuating hole cut punch 10. In addition, FIG. 5 illustrates the operative state of the right-hand side R of the injection molding die wherein nozzle bush 11 is displaced in a rearward direction, and at the same time, the hole cut punch 10 is displaced in a forward direction, causing a center hole 13 to be formed through the molded product of base board. Also in this case, since the center hole 13 is formed through the base board by displacing the hole cut punch 10 in the forward direction while displacing the nozzle bush 11 in the rearward direction under no load, there arises a problem that the center hole does not exhibit a smooth appearance but it exhibits a coarse appearance similar to that formed by forcible breakage caused by a punching process.
The inventors of the present invention conducted a variety of research and development experiments for improving the accuracy of a double refraction index, and invented a re-compressing type molding method (patented under U.S. Pat. No. 4,715,804) wherein a molten resin is injected in a cylindrical die cavity, and thereafter, a molded product of base board is re-compressed by tightening both die halves of an injection molding die. FIG. 6 and FIG. 7 show the injection molding die which was employed for practicing the re-compressing type molding method proposed by the inventors. FIG. 6 is a plan view of the injection molding die, and FIG. 7 is a sectional view of the injection molding die wherein the left-hand side L of the drawing illustrates the operative state of the injection molding die after completion of a primary molding step, and the right-hand side R of the drawing illustrates the operative state of the same when a molded resin of base board molded by the primary molding step is re-compressed.
A characterizing feature of the re-compressing type molding method consists in that a density of each base board can be increased by re-compressing a molded product of base board molded by the primary molding step while the die cavity is kept closed. Moreover, a double refraction index does not fluctuate by uniforming the density of the filled molded product of base board over its entire area. Another characterizing feature of the same consists in that a dimensional accuracy, a surface roughness, and a flatness of a center hole formed through the base board can be improved by employing the re-compressing type molding method.
In FIG. 7, reference numeral 20 designates a die half on the stationary side of an injection molding die for molding base boards. As is apparent from the drawing, the die half 20 is constructed as a die assembly fixedly secured to a base plate 21. Reference numeral 22 designates four column-shaped guide posts each serving to support a die tightening pressure.
A die half 23 on the movable side of the injection molding die is likewise constructed in the form of a die assembly held on a movable plate 25. Four guide bushes 24 are press-fitted through the movable plate 25 in an axial direction so as to allow the movable plate 25 to slidably move along guide posts 22. Die half 23 can be displaced in an axial direction by a hydraulic unit 27 arranged in a base plate 26 on the movable side using re-compressing. To accomplish this, a piston 28 of the hydraulic unit 27 is operatively connected to the die half 23 via the baseplate 26.
As shown in FIG. 7, while the base plate 21 is fixedly secured to a platen 29 of an injection molding machine (not shown), and the base plate 26 is fixedly secured to a platen 30 of the same, a molten resin is injected into a die cavity 31 through an injection hole 33 formed through a nozzle bush 32 to execute a primary molding step. On completion of the primary molding step, a primarily molded product 34 is obtained within the injection molding die composed of the die half 20 and the die half 23. At this time, a distance H between the platen 29 for the die half 20 and the platen 30 for the die half 23 on the movable side is kept constant during the primary molding step with the aid of the four guide posts 22.
On completion of the primary molding step, a gap t corresponding to a distance corresponding to the recompression, is reserved between the die half 20 on the stationary side and the die half 23 on the movable side so as to execute a secondary molding step. Thus, when it is assumed that a final thickness of each base board is represented by t, the thickness of the primarily molded product is represented by t+.DELTA.t. Usually, the gap .DELTA.t is set to 10 to 20% of the final thickness t.
Subsequently, a secondary molding step is executed. The piston 28 in the hydraulic unit 27 is driven in the upward direction with a predetermined recompression pressure so that the die half 23 is brought in tight contact with the die half 20, causing the gap .DELTA.t to be reduced to zero, whereby a molded product of base board 35 is obtained.
After completion of the secondary molding step, a center hole is formed through the base board 35 by actuating a hole cutting punch (not shown). Subsequently, the die half 23 is separated from the die half 20 so as to allow molded product of base board 35 to be removed from the injection molding die, thus completing a single molding cycle.
With the injection molding die constructed in the above-described manner, the filled density of a molded product of base board can be increased by re-compressing a primarily molded product after completion of the primary molding step. However, this known injection molding die has a few problems as noted below.
According to the prior invention, four guide posts 22 are arranged for the injection molding die as die tightening pressure supporting members. However, it has been found that the center axis of both die assemblies is not positionally coincident with the center axis among the four guide posts 22 due to temperature differences unavoidably arising between both die assemblies (attributable to the fact that the temperature of the die assembly on the stationary side is usually kept higher than that of the die assembly on the movable side), resulting in some positional error being induced with the conventional injection molding die. In addition, the guide bushes 24 are forcibly fit into the movable plate 25 while the foregoing positional non-coincident state is maintained, causing an excessively high intensity of bending stress when the guide posts 22 are fitted through the guide bushes 24. This results in the fitting surface of each guide bush 24 being readily damaged or injured.
As mentioned above, according to the prior invention, each molding operation is performed using the primary molding step, the secondary molding step accompanied by re-compressing, and the formation of a center hole through the molded base board. However, since the secondary molding step accompanied by the re-compressing is executed while the nozzle bush in communication with the die cavity, the compressive stress induced in both die assemblies during the secondary molding step is transferred to the nozzle bush, resulting in the uniformity of the density of an inner periphery of the molded product of base board being adversely affected.
In addition, since it is necessary that the injection molding machine be equipped not only with a hydraulic unit exclusively employable for achieving the re-compressing but also with a hydraulic unit exclusively employable for forming the center hole through each base board, a sequence of actuation of the two hydraulic units and an actuation time of each should separately be controlled, resulting in a controlling unit for the injection molding machine becoming unavoidably complicated in structure.